Water based photoresists based on natural products such as fish glue, albumin and casein, have been known for a number of years. A useful photoresist composition comprises casein, made by acid precipitation of milk, an alkali metal base to impart a pH of 7.7 or higher which improves the solubility of the natural product in water, and an ammonium dichromate sensitizer.
U.S. Pat. No. 4,061,529 of Goldman and Datta, herein incorporated by reference, discloses the addition of sodium borate as the base in a concentration so that the resist solution has a final pH of 6.7-7.3, to improve the pot life of the coating composition and the shelf life of coated substrates.
These resists have been employed extensively in the lithographic printing arts and in the manufacture of shadow masks for television monitors. A cleaned metal surface or substrate is coated with the aqueous photoresist and dried. A mask having a desired pattern is contacted to the resist and exposed to ultraviolet light which hardens the resist in the exposed areas. The resist film is then washed with water to dissolve the unexposed resist and thereby uncover part of the metal substrate. The now partially coated substrate is then dried and baked for about 5 minutes at a temperature of from 500.degree.-550.degree. F. (260.degree.-287.degree. C). This step is required to render the remaining photoresist etch resistant. The partially coated substrate is then etched by spraying with hot ferric chloride solution which etches away the bared metal portions. The residual resist can then be removed in hot alkali solution.
The above process has been universally adopted for television shadow mask production because of its low cost, e.g., the photoresist, water rinse and caustic wash solutions are very inexpensive as compared to organic based photoresists.
However, this process has certain limitations which prevent its use in other high production manufacture which employ lithographic techniques. In particular, in the manufacture of printed circuit boards, a copper clad phenolic impregnated paper board is etched to form a pattern of conductors to which various components are soldered.
Generally, printed circuit boards are made by screen printing an etch resistant ink onto the boards and etching the exposed metal areas. However, this method is not able to define line widths of less than about 0.025 inch (0.06 cm). With the increasing miniaturization of components on printed circuit boards, the need to define line widths and spacings on the order of 0.015 inch (0.04 cm) and smaller has occurred. Photolithographic techniques to define such fine pattern spacings must then be employed.
The pattern is made by applying a photoresist film over the copper layer, exposing and developing the resist to create a pattern of photoresist and exposed copper. The copper is etched away in the exposed areas and the photoresist is then removed, leaving a patterned copper layer on the board. At present, organic based photoresists are used because the high temperatures required to cure water based resists cannot be tolerated by the phenolic impregnated paper substrate.
The water based photoresists have other deficiencies as well for this application; they are subject to premature hardening, called dark hardening, particularly under high humidity conditions, and they have a limited shelf life.
Thus an inexpensive, sensitive water based photoresist having low cure temperatures and improved shelf life would be highly desirable for defining fine patterns on printed circuit boards.